First of all, it is necessary to understand that during the war there were two distinct “families” of Mosquitoes, with different Merlin engine marks fitted to the different families.
The early variants were powered by single stage, two speed Merlin 20 family engines (21, 23 and 25s off the top of my head).
With these engines their max speeds topped out at about 360-385 mph, depending on the exact engine type and external fittings, armament, weight, drop tanks ect, ect, ect.
The Merlin 20 engine types were fastest at low to medium altitudes, with altitudes for best level speeds ranging from 14,000 to 22,000 feet. Sea level speeds ranged from 310 mph to 355 mph, getting progressively faster during the war as more powerful engines with higher permitted boost ratings were allowed.
The second “family” were powered by the two speed, two stage Merlin 60/70 engines (61, 71, 72, 73, again IIRC), and were introduced later in the war (early to mid 1943 if memory serves correctly).
These were more specialised for high altitude work, with typical best speeds of 395-415 mph at 24,000 to 29,000 feet.
The Mosquito in Aces High is a FB VI, a fighter bomber with a top speed of about 375 mph
Secondly, the combat environment in flight simulators is not necessarily the same as that in the real world. Mosquito pilots could, and did, outrun enemy single seat fighters (FW 190s/Bf 109s) in level flight, at both high and low levels. But considerations such as endurance, weather, engine condition et al are unimportant in a flight simulator, whereas they are very important in the real world.
Even though the Mosquito was fast, there was no guarantee that it could simply outrun its opponents. There are multiple accounts of Mosquitoes being overhauled by German fighters in a stern chase and shot down, particularly at lower altitudes. There is even a German account of an FW-190 having its base bombed by Mosquitoes, taking off in pursuit and then shooting down the bomber after a chase.
As the war progressed, the relative performance of the Mosquito at low altitude was overhauled by progress with German fighters. Where the Mosquito came to be almost immune, and where Merlin 60/70 powered marks spent most of their time, was above 25,000 feet and at night.
If a Mosquito is cruising at excess of 330-350 mph (twice what a Lancaster or B-17 would cruise at), at 27,000 ft or higher, then the problem for German interceptors is massively multiplied. Not only do they have half (or less) of the time to track, predict and locate the target of a Mosquito bomber, they then have to get their fighter up to that altitude, which generates another penalty in terms of time, and also in fuel burnt (and hence range to intercept).
This is where the Mosquito was in its element.
With the FB Mk VI, a low-medium altitude fighter-bomber, the interception picture is a little easier for its opponents. Additionally, the performance differential is in favour of the German aircraft, particularly once the later G model 109s and the FW 190A5 are introduced.
